Organic backing



May 6, 1952 (Preferable) Spray Primer Coat L. D. MQGRAW ETAL 2,595,867

ORGANIC BACKING Original Filed May 25, 1950 Deposition of LightReflecting Layer on Light Transmitter FIGURE l Styrene Butadiene BaseComposition Light Reflecting Layer Light Transmitter FIGURE 2 tyreneButodiene Composition Coat Light Reflecting Layer Light TransmitterFIGURE 3 INVENTORS. Leslie D. McGraw BY Charles L..Faii'st Ernest R.Mueller W/ W MM AGENTS.

Patented May 6, 1952 oncnnrc BACKING V Leslie D. McGraw, Charles L.Faust, and Ernest R. Mueller, Columbus, Ohio, assignors, by mesneassignments, to Ohio Plate Glass Company, Toledo, Ohio, a corporation ofOhio Original application May 25, 1950, Serial No. 164,057. Divided andthis application May 25, 1950, Serial No. 164,058

(Cl. 260--i1,5)

2 Claims.

This invention relates to protective backings or coatings for mirrors.More particularly, this invention relates to coating compositions forprotecting the backs of mirrors, to a mirror coated with suchcompositions, to a mirror having a composite backing, and to a method ofprocess for protectively coating mirrors.

While many compositions have been proposed in the prior art as backingmaterials for mirrors, they have been found to seriously lack thenecessary protective ability to prevent deterioration or oxidation ofthe silver layer of the mirror over a long period of time. Thesematerials sometimes contain residual corrosive agents or generate thesame during exposure to the moisture-laden atmosphere resulting inattack or oxidation of the silver film, thus ruining the mirror.Likewise, these materials may crack or pit on aging to allow the silverto blacken, or white spots to appear, as well as permitting moisture topenetrate to the silver film and also deteriorate it. In industry it isdesirable to produce mirrors in large sheets which are subsequently cutinto commercial and retail sizes. It is essential that, during thecutting process, the protective backing material not adhere or stick tothe cutting tool or abrasive wheel, and also that it not permit flakingor peeling of the silver film fromthe glass base. Likewise, when theedges of the mirror are finished or polished, the protective backingshould wear awayreadily and should not cause the silver film to pullaway from the edge. Another desirable featurein a commercial mirrorbacking is that it be sufficiently hard and tough to resist scratchingor marring during assembling, handling and mounting. A still furtherproperty of a good mirror backing relates to its ability to preventdeterioration of the silver if there is an accidental scratch throughthe backing which exposes a small area of the silver. These propertieshave not heretofore been obtained adequately with organic backings thatare available today, result ing in high cost of mirror manufacture andshortened life. Hence, it would be highly desirable to obtain a tough,readily-friable, moisture-impervious, noncorrosive and adherent mirrorbacking. A

It is, therefore, an object of this invention to provide a compositionof matter which will protect the reflecting film of a mirror againstdeterioration, oxidation by the atmosphere, flaking or loosening of thefilm during cutting, and marring or scratching when handled andassembled in place.

It is another object of this invention to provide a method or processfor coating a mirror 2 with a protective composition characterized bythe 'fact that said composition protects the mirror againstfiaking,peeling, and scratching during manufacturing and finishing-operationsand against oxidation and deterioration in use.

It is yet another object to provide a mirror having an adherentscratchand mar-resistant, nonflaking protective composition backingwhich materially increases the life of the mirror.

' It is a further object of this invention to provide a mirror having acomposite backing which is characterized by having the aforementionedimproved qualities.

It is a still further object of this invention to provide a method forproducing a mirror having a composite backing which is characterized byhaving the aforementioned improved properties.

These and other objects and advantages of the present invention willbecome more apparent from the following detailed description, example,and drawing wherein,

Figure 1 is a flow sheet of the method or process disclosed herein, andi Figure 2 is a vertical cross-sectional view of the mirror of thisinvention having the protective coating composition thereon, and

Figure 3' is a vertical cross-sectional view of the mirror of thisinvention having the protective composition and a primer coat thereonand which represents one perferred embodiment of the present invention.

' the reflective layer or surface of the mirror and provides, on dryingand hardening, a backing or outer {coating which prevents flaking 01.loosening of the reflective layer during manufacturing operations suchas cutting, trimming or tapering, and resists mars and scratches duringhandling and deterioration by oxidation over relatively long periods oftime. Moreover, the life of the mirror can be improved by disposing aprimer coating between this outer backing and the reflective silver lessfilm of the mirror.

In general, with reference to Figures 1 to 3 of the drawing, it will beseen that a mirror made by any well-known process, for example, such asthe Rochelle salt'chemical reduction process, is simply sprayed with amixture of the aforementioned ingredients and then allowed to airdry.The mirror can also contain an intermediate organic base primer coatdisposed between the silver film and the outer protective composition byspraying, etc., when it is desirable to obtain a somewhat greater lifeand adherence of the other protective coating to the mirror.

The light-reflecting material has been described as a mirror although itis obvious that reflectors, semior selectively reflecting or evenpartially or semi-transparent, can be treated according to the inventiondisclosed herein in order to increase their life. as Well as to protectthem against injury during manufacturing and assembly operations. Thelight-permeable or transmitting means of the mirror ispreferably glass,but plastic, cellophane and similar lighttransmitting or semiorselectively light-transmitting materials may likewise be used which willnot be adversely affected by a temperature up to: 120 C.The'light-refiecting. means on the mirror-can be of any metallic.orother'.sirnilar substance. For the purposes of this invention thingold, copper, platinum, and silver films are preferable. It is onlynecessary to have a very thinfilm in order to obtain a good mirror. Thesilver or other metal may be deposited by processes of spraying, meltingand puddling, vacuum evaporation, sputtering or sublimation, dipping, orby chemical reduction processes such as the Rochelle salt process. TheRochelle salt process is probably the best known process for producinggood mirrors and is preferred in view of the fact that it deposits-avery thin, continuous, highlyreflectivefilm of metal which is veryadherent to the glass.

It: is not precisely known why the combination of elements of theprotective composition of this invention provides such a good coatingmaterial. However, it has been found that outside certain ranges orproportions of these materials, the resulting composition will not havethe necessary protective properties or qualities as taught by thepresent invention. In the composition the critical ranges are asfollows: the zinc oxide should be used in the amount of from 13 to 33parts by weight, barytes in an amount of from 16 to 40.5 partsby weight,talcfrom 3.0 to 15.5 parts by weight, titanium dioxide from 6.0 to 11.5parts by weight, styrene-butadiene co-polyrner in an amount of from 25to 32 parts by weight, and sufiicient xylene to give good fiowability orviscosity for coating by painting, spraying, etc.

The amount of xylene used will vary depending on the fluidity orviscosity desired during mixing and spraying and is generally used inthe ratio of from 60 to 120 parts by weight of xylene to about 30 to 60parts by weight of the mixture of dry ingredients. Aryl hydrocarbonssimilar to xylene, such as toluene and benzene, can be substitutedpartially or wholly for the xylene.

A representative backing composition contains 16 parts by weight of zincoxide, 25.7 parts by weight of barytes, 3.2 parts by weight talc, 6.1parts by weight of titanium dioxide, 26.3 parts by weight ofstyrene-butadiene co-polymer and 136 parts by weight of Xylene. Xyleneis usually added to the styrene followed by addition of the otheringredients. Additional xylene is added as needed to vary the viscosityand enable the composition to be readily sprayed. The composition afterevaporation of the xylem procl'u'ced an excellent backing from 0.5 to1.0 mil thick when applied to mirrors.

A primer coat is desirably added to the silver film prior to thedeposition of the outer organicbase protective backing. Any primercoating Components A, Dose Grind:

Vinyl Butyrel Resin XYHL (polyvinyl butyral resin-as manufactured by theBakelite Co.) 7. 2

Basic Zinc Chromate pigment or zinc tetroxy chroznate (M 1828-MineralPigment Corp. or 2259- Imperial Paper and Color 00.)

Talc (Asbestiuc 3X) Isopropanol (99%) olueue B. Acid Diluant:

Phosphoric Acid W" tor a Isopropanol (99%) The above ingredients aremixed prior to use. The mixture should, however, be reduced forapplication and used within eight to twelve hours after preparation toprevent excessive polymerization and evaporation of solvent and.diluent, e. g., water, isopropanol, and toluene. It is obvious that theproportions can be varied somewhat to still obtain desirable adherenceand moisture-resistance properties which are necessary in helping toprevent deterioration of the silver film.

Any paint or rubber-mixing machinery can be used to mix the ingredientsof the primer coating and also the outer coating. For example, a ballmill or three-r011 paint mill can readily be employed. The mixingprocess is discontinued at the time a homogeneous-appearing mixture isobtained which is in from about 20 to 40 minutes. The time will, ofcourse, vary with the size of the equipment and amount of materialsbeing used.

Customary paint-spraying equipment can be readily used to apply theprimer and outer coatings. t is unnecessary to use specialpaintspraying' machinery or techniques in order to coat the mirror withthe compositions disclosed herein. Moreover, it is not necessary thatthe coatings be only sprayed but they can be readily pplied byrollercoating, dipping, brushing, and so forth.

The air-drying and hardening step to enable the volatiles to evaporateand the polymersor resins to set may be done in the open atmosphere atroom temperature. While the coatings are dry in about 45 minutes in air,they are not sufficiently hard for several hours. It is not criticalthat the coatings be dried in a stream of hot inert gas or heated,although such procedure can be utilized to materially reduce the time ofdrying. For example, it will take about 12 to 24 hours to dry and hardenin air at room temperature while baking at '75-100 C. will take onlyfrom 30 to GO'minutes. It is, of course, apparent that when a primercoat is used, it must be allowed to dry and set before the outer organicbase backing is applied.

The thicknesses of the primer and outer coatings can, of course, bevaried by spraying, etc., for longer times or by repeating theprocedures as often as desired.

While it is not essential, it is desirable that the materials used inthe practice of the present invention be substantially pure in order toobtain consistent results. Unless otherwise specified, the Weightsexpressed in the specification and claims are parts by weight.

Thefollowing example will serve to illustrate the invention with moreparticularity to those skilled in the art.

14 parts by Weight of acicular zinc oxide meeting Maritime Commissionspecification 25-MC- 520, 16 parts by weight of medium oil-absorptionbarytes (approximately 99% BASO4-0.1 to 0.025 retained on 325 meshscreen), 8 parts by Weight of high oil-absorption talc (1.2 to 1.3%retained on 325 mesh screen), 6.5 parts by weight of rutile titaniumdioxide (95% TiOz meeting Federal specification TT-T-425, Type III,class B), and 25 parts by weight of styrene-butadiene co-polymer(Pliolite S-5 as made by the Goodyear Tire and Rubber Co. 70 to 75%styrene). Xylene was added to the styrene prior to addition of otheringredients in the amount of 106 parts by weight. Next the componentswere mixed together on a three-roll mill until a homogeneous-appearingcomposition was obtained. This took about thirty-five minutes. Thematerial was then put in a commercial paint sprayer and sprayed on thesilvered back of a mirror to produce a, protective backing 0.9 milthick. After 24 hours in air at room temperature, the backing was dryand hard.

Other examples were run in which the primer coat described supra wasdisposed between the silver film and the outer protective coating.

The tests below show the results of using the protective coating of thisinvention for mirror backings as compared to mirrors containing ashellac coating and containing no protective coating.

Test I A mirror containing only the protective outer organic coatingdescribed in this application on its back lost its brilliance withinthree days under an atmosphere of 100% humidity at 125 F.

Test II A shellac backing applied to a silvered mirror Without anintermediate coating offered protection for less than one day whenexposed to an atmosphere of 100% relative humidity at 112 F.

Test III With no protective coating the silvered mirror lost itsbrilliance and adherence in one day at 112 F., 100% relative humidity.

Test IV the primer coat, was not readily scratched or marred, nor oncutting did the silver film flake or peel from the glass.

It is, thus, seen that the present invention discloses a new and novelprotective-coating composition for minors containing zinc oxide,barytes, talc, titanium dioxide, butadiene-styrene co-polymer andxylene. This composition can be readily applied to the reflective back,layer or surface of the mirror and on drying resists atmospheric oxygenand moisture and thereby protects the reflective surface againstdeterioration. Moreover, during manufacturing and mounting operations,such as grinding, finishing or cutting, the reflective film does notflake or peel from mirrors containing the protective-backing compositionof this invention. Additional improved results in adherence of the outercoating and resistance in deterioration of the silver film are obtainedwhen a primer coating or layer is disposed bet on the metallic film andthe outer organic base acking material.

It will be understood that while this invention has been described withparticular reference to protective coatings for mirrors, and morespecifically to silvered mirrors, it is to be understood that in itsbroad concept the process and the novel composition disclosed herein areapplicable to any device containing reflecting or semi-reflectingsurfaces which are subject to oxidation or deterioration, scratching,flaking during handling or manufacture, or after mounting or use, and,thus, the matter disclosed herein is to be construed as illustrativerather than limiting.

This application is co-pending with and related to McGraw, Faust, andMueller application, Serial No. 164,057, filed May 25, 1950. The presentcase is a divisional application of application, Serial No. 164,057,which is directed to the production of a mirror or composite articlehaving a laminated backing composed of a number of organic and inorganiccoatings. The organic coating can be the organic-coating claimed in thiscase.

Having thus described theinvention, what is claimed as new and novel andis desired to be secured by Letters Patent is:

1. A composition of matter, comprising from 13 to 33 parts by weight ofzinc oxide, from 16 to 40.5 parts by weight of barytes, from 3.0 to 15.5parts by weight of talc, from 6.0 to 11.5 parts by weight of titaniumdioxide, from 25 to 32 parts by weight of styrene-butadime co-polymercontaining to styrene and xylene in an amount sufficient to give coatingviscosity.

2. A composition of matter, consisting of 16 parts by weight zinc oxide,25.7 parts by weight barytes, 3.2 parts by weight talc, 6.1 parts byweight titanium dioxide, 26.3 parts by weight styrene-butadieneco-polymer containing 70 to 75% styrene and xylene in an amountsufiicient to give coating viscosity.

LESLIE D. MoGRAW. CHARLES L. FAUST. ERNEST R. MUELLER.

REFERENCES CITED The following references are of record in the file ofthis patent:-

UNITED STATES PATENTS Number Name Date 2,294,940 Skolnik Sept. 8, 19422,394,930 McRae Feb. 12, 1946 2,410,733 Hewlett Nov. 5, 1946 2,419,512Vesce Apr. 22, 1947 2,459,891 Nelson et al. Jan. 25, 1949 2,468,568MoCusker Apr. 26, 1949 2,502,949 Howlett et al. Apr. 4, 1950

